#include <stdio.h>
#include <math.h>

int plus(int number){
    return number*(number+1)/2;
}
int minus(int number){
    return (-(number*(number+1)/2))+number;
}
int main(){
    int N;

    scanf("%d",&N);
    if(abs(N)>=1  && abs(N)<=10000){

        if(N<0)
            printf("%d",minus(N));
        else
            printf("%d",plus(N));
    }


}